Many different approaches have been made to the problem of monitoring and indicating the discharge condition of electric storage batteries, especially in applications where the battery is first charged and then is used in the discharge mode for a considerable period of time, such as in a mobile vehicle, before it is again returned to the charger to be re-charged.
Various battery terminal voltage measurement systems have been employed for the purpose of monitoring battery discharge, with varying degrees of success. The present invention is essentially in this category.
One approach is simply to measure and read the instantaneous battery terminal voltage on a continuing basis. However, this requires interpretation, since the terminal voltage varies during operation of the apparatus powered by the battery, having a depressed value during loading, dependent upon the magnitude of the load.
Various unidirectional systems have been devised for detecting and registering downward excursions in battery voltage under load, as an indication of the discharge condition of the battery. Some of these have operated on the basis of recognition of a sustained undervoltage condition before any registration is made, and others are really undervoltage analyzers which recognize and register undervoltage conditions in a setting recognizing previously registered undervoltage conditions. Very accurate results have been obtained with such systems especially when applied to apparatus having known patterns of loading, even where the loading is quite discontinuous, such as in the operation of industrial fork lift trucks. Examples of successful systems of this sort are disclosed in a prior U.S. Pat. No. 4,193,026 issued to Eugene P. Finger and Eugene A. Sands on Mar. 11, 1980 and assigned to the same assignee as the present application.
The above mentioned devices are sometimes referred to as "fuel gauges" since they are intended to indicate how much operating capacity remains in the battery of the vehicle. These devices are intended to be continuously connected to the vehicle or apparatus whose batteries are being monitored so as to analyze the operating history of the battery as a basis for a continuing calculation and indication of the remaining charge on the battery.
There is an important need for a battery tester which is capable of easily and conveniently providing an indication of the charge state of a battery without the necessity for having the device constantly connected to, or associated with, the battery. Such a tester would be especially valuable in a battery charging facility where it may be desired to determine the charge state of batteries which are held in temporary storage to make certain which batteries have been charged, and whether they have been fully charged or not, or to determine how much charge they need if they are not charged. Such a tester or testing method would also be useful for the purpose of determining the charge state of batteries in an electric vehicle or other apparatus which is not equipped with a "fuel gauge" of sufficient accuracy, or which has no "fuel gauge".
One of the most common present methods for determining the state of charge of batteries in a test procedure is by checking the specific gravity of the fluid electrolyte. This means removing the cap from each battery cell, inserting the electrolyte tester tube into that cell, drawing electrolyte liquid into the tester, waiting until the fluid level is steady, and then attempting to read the position of the meniscus of the fluid in the tester in relation to a float scale to provide a specific gravity reading. The fluid must then be returned to the cell and the cell cover again closed. This procedure must be repeated eighteen times for a thirty-six volt battery to have a complete reading.
Even if a sampling procedure is used, in which three or four cells are tested, and the others are assumed to be the same, the procedure is still very time consuming and the accuracy of the reading is seriously limited. Another very serious problem with the accuracy of the reading arises from the fact that it is only the stabilized steady state specific gravity of the electrolyte which is an accurate indication of the state of charge of the battery. Thus, if the battery has been in a non-quiescent condition (connected for either charge or discharge) for less than a number of hours before the specific gravity is taken, the specific gravity reading is not an accurate indication of the remaining state of charge. Many hours are required for the specific gravity to stabilize sufficiently to provide a reasonably accurate reading. Furthermore, there is always a risk of spilling the battery acid electrolyte. Accordingly, the conventional specific gravity measurement has a number of serious disadvantages.
There is also an important need for a battery tester which is simply capable of quickly providing an indication as to whether a battery was last subjected to a charge or discharge operation. In a battery charging facility, this will provide a quick confirmation as to which batteries have already been charged, and which batteries still need to be charged.